Liquid cooled transformers are conventionally filled with a mineral oil. Due to environmental benefits and fire safety properties, more easily bio-degradable oils such as natural or synthetic esters and other synthetic liquids are used to substitute the traditional mineral oil. There is an increasing demand for refilling i.e. exchanging the mineral oil of an existing transformer to a less flammable liquid.
The refilling procedure comprises more than simply delivering the new liquid and replacing the old one. The procedure also needs to include the assessment of the transformer and its expected performance after the oil exchange. The thermal behaviour of the transformer will be changed due to different thermal properties and different viscosity of the new liquid. Since the used and aged transformers are initially designed for mineral oil and eventually used with mineral oil, it needs to be assessed whether the transformers still satisfy their operation requirements after the oil exchange.
In cases of old transformers there is often only limited information such as nameplate information, test reports and external dimensions available. The transformers may come from different manufacturers and comprise different technologies and designs. Normally there is no access or a restricted access to the active parts i.e. the core and the windings of the transformer. Consequently, the use of conventional design tools may not help to estimate the thermal performance of these transformers.
EP1786083 discloses a method for monitoring power transformers. Failures are detected in a transformer by comparing measured values obtained from a thermal model.